The invention relates to a technique for driving a liquid jetting head of a liquid jetting apparatus such as an ink jet printer, wherein ground-side electrodes of piezoelectric elements provided in the liquid jetting head so as to be associated with nozzle orifices to be used for jetting liquid droplets, are held at a predetermined bias voltage.
Various kinds of the liquid jetting apparatus have hitherto been known. For example, there have been known an ink jet printer which records information on recording paper by jetting ink droplets, an electrode forming apparatus which forms an electrode on a board by jetting liquid-state electrode material, a biochip manufacturing apparatus which manufactures a biochip by jetting biological specimen, and a micropipette for jetting a predetermined amount of sample into a vessel.
Ink jet color printers which eject color ink from a recording head have become pervasive as computer output devices. The printers are widely used for printing images, which have been processed by a computer, in multiple colors and gradations.
For instance, in an ink jet printer employing piezoelectric elements as drive elements for ejecting ink, a plurality of piezoelectric elements, which are provided so as to correspond to a plurality of nozzle orifices of a print head, are selectively activated, whereby ink droplets are ejected from the nozzle orifices in accordance with the dynamic pressure developing in the respective piezoelectric elements. Dots are formed on print paper by causing the ink droplets to adhere to the print paper, thus effecting printing operation.
Here, the piezoelectric elements are provided so as to correspond to nozzle orifices to be used for ejecting ink droplets. The piezoelectric elements are actuated by a drive signal supplied from a driver IC (i.e., a head drive circuit) mounted in the print head, thereby ejecting ink droplets.
Such a head driver is configured in a manner shown in FIG. 4.
As shown in this figure, a head driver 1 comprises piezoelectric elements 2 provided so as to correspond to respective nozzle orifices; a head drive circuit 3 to be used for supplying a drive signal to a first electrode 2a of each piezoelectric element 2; and a current amplifier 4 and a switcher 5, both being interposed between the head drive circuit 3 and the respective piezoelectric elements 2.
The piezoelectric elements 2 are configured so as to become deformed by a voltage applied across electrodes 2a and 2b. 
The head drive circuit 3 provides a drive signal COM to the head of the ink jet printer and is provided in, e.g., a printer main unit.
The current amplifier 4 is constituted of two transistors 4a, 4b. Of the transistors, a collector of the first transistor 4a is connected to a constant voltage source, and a base of the same is connected to one of outputs of the head drive circuit 3. Further, an emitter of the transistor 4a is connected to an input terminal of the switcher 5. In accordance with the signal output from the head drive circuit 3, a constant voltage Vcc is supplied to the piezoelectric elements 2 by way of the switcher 5.
An emitter of the second transistor 4b is connected to the input terminal of the switcher 5, and a base of the same is connected to a second output terminal of the head drive circuit 3. Further, a collector of the second transistor 4b is grounded. As a result, the second transistor 4b is brought into conduction in accordance with a signal output from the head drive circuit 3, thereby causing the piezoelectric elements 2 to discharge by way of the switcher 5.
Upon receipt of a control signal, the switcher 5 is activated at a timing at which a corresponding piezoelectric element 2 is activated, whereby the drive signal COM is output to that piezoelectric element 2.
In fact, the switcher 5 is constituted of a so-called transmission gate to be used for activating and deactivating the respective piezoelectric elements 2.
Here, the head driver 1 having such a configuration often causes an anomaly, such as a short-circuit or a rare short-circuit, for reasons of a failure arising in any piezoelectric element 2 or a like reason. In such a case, when the constant voltage Vcc is applied to the piezoelectric elements 2 by way of the switcher 5 as a result of the first transistor 4a of the current amplifier 4 being turned on, an anomalous current flows into the piezoelectric elements 2 through the constant voltage Vcc.
Therefore, in order to prevent flow of the anomalous current into the piezoelectric elements 2, which would otherwise be caused by the constant voltage Vcc, a fuse 6 has hitherto been interposed between the constant voltage Vcc and the collector of the first transistor 4a. 
In the event that a short-circuit or rare short-circuit has arisen in any of the piezoelectric elements 2, if an anomalous current flows into the head driver 1, the fuse 6 is disconnected by the anomalous current, thereby preventing flow of the anomalous current into the piezoelectric elements 2.
When such piezoelectric elements remain in a non-actuated state (i.e., when printing is not performed), the electric charges accumulated in the piezoelectric elements as a result of charging operation are discharged by insulation resistance, whereby the voltages of the piezoelectric elements are lowered, possibly affecting ejection of ink.
Therefore, there has also been known a head driver, in which ground-side electrodes of respective piezoelectric elements are held at a bias potential; for example, an intermediate potential of a drive signal. Such a head driver is configured as shown in FIG. 5.
As shown in this figure, a head driver 7 is substantially identical in configuration with the head driver 1 shown in FIG. 4. A capacitor 8 serving as a charger to be charged by a constant voltage Vc1 is connected to a second electrode 2b in each of the piezoelectric elements 2.
As a result, the remaining electrodes 2b of the piezoelectric elements 2 are held at a bias voltage originating from the capacitor 8 serving as charger. The voltage developing across the electrodes 2a, 2b of the piezoelectric elements 2 is diminished, thereby preventing discharge, which would otherwise arise in electrodes of the piezoelectric elements when packing density of the piezoelectric elements is increased.
However, as mentioned previously, when a short-circuit or rare short-circuit has arisen in each of the piezoelectric elements of the head driver 7 of such a configuration, a bias voltage originating from the capacitor 8 serving as the charger is applied to the remaining electrodes 2b of the piezoelectric elements 2, whereby the voltage across the electrodes 2a, 2b of the piezoelectric elements 2 is lowered. Hence, if an anomalous current flows from the constant voltage Vcc, no sufficiently-large anomalous current will flow. As a result, blowing of the fuse 6 is prevented, whereupon the anomalous current flows into the piezoelectric elements 2.
For this reason, a print head is equipped with a temperature detector, thereby detecting a rise in the temperatures of the piezoelectric elements 2 induced by an anomalous current.
It is an object of the invention to provide a head driver for a liquid jetting apparatus arranged so as to reliably detect occurrence of short-circuit or rare short-circuit in a piezoelectric element.
In order to achieve the above object, according to the invention, an anomalous voltage arising in ground-side electrodes of piezoelectric elements is detected on the basis of a charging voltage of a charger for applying a bias voltage to the ground-side electrodes of the piezoelectric elements.
Specifically, there is provided a head driver for a liquid jetting apparatus, comprising:
a head drive circuit, adapted to selectively drive piezoelectric elements for ejecting liquid droplets from a liquid jetting head;
a charger, adapted to apply a bias voltage to ground-side electrodes of the piezoelectric elements; and
an anomalous voltage detector, which outputs a detection signal when a charging voltage of the charger is a predetermined value or more.
In such a configuration, in the event that an anomaly, such as a short-circuit or rare short-circuit, has arisen in the piezoelectric elements, an anomalous current flows into the charger by way of the piezoelectric elements. The charger is further charged by the anomalous current. Since the charging voltage of the charger is eventually increased, the anomalous voltage detector outputs the detection signal when the charging voltage becomes the predetermined value or more.
Accordingly, a control section of a liquid jetting apparatus can detect occurrence of an anomaly, such as a short-circuit or rare short-circuit, arising from a failure in the piezoelectric elements of the head driver, in accordance with the detection signal output from the anomalous voltage detector.
Further, upon receipt of the detection signal from the anomalous voltage detector of the head driver, the control section may controls the head driver so as to temporarily suspend the liquid jetting operation. On some occasions, the control section forcefully terminates the liquid jetting operation, thereby preventing destruction of the liquid jetting head, which would otherwise be caused by an anomalous current flowing through the piezoelectric elements.
In this way, in the event that an anomaly, such as a short-circuit or rare short-circuit, arising from a failure in the piezoelectric elements has arisen, the head driver of the invention can detect an anomalous voltage attributable to an anomalous current flowing through the ground-side electrodes of the piezoelectric elements even when the bias voltage such as an intermediate potential is applied to the ground-side electrodes of the piezoelectric elements.
In this case, an anomalous voltage developing in the ground-side electrodes of the piezoelectric elements can be detected through use of a simple configuration embodied by addition of only the anomalous voltage detector for monitoring the charging voltage of the charger to a related-art head driver. Hence, a fuse provided for a constant voltage is obviated, thereby diminishing costs of components.
Preferably, the anomalous voltage detector includes a switching element which changes a conduction state thereof when the charging voltage of the charger is a predetermined value or more.
In such a configuration, a threshold value of the charging voltage of the charger to be used for activating or deactivating the switching element is set so as to become slightly higher than the charging voltage of the charger employed during normal operating conditions. As a result, an anomalous current flows into the piezoelectric elements. When the charging voltage of the charger has increased, the switching elements are activated or deactivated, whereupon a detection signal is output while being reversed from a high level to a low level or vice versa.
More specifically, the anomalous voltage detector divides the charging voltage of the charger by a voltage-dividing resistor, thereby producing an appropriate voltage. The switching element, such as a transistor, is turned on or off by such a voltage. Therefore, an anomalous voltage developing in the ground-side electrodes of the piezoelectric elements can be detected through use of a considerably simple configuration embodied by addition of a mere voltage-dividing resistor or switching element to the related-art head driver.
Alternatively, it is preferable that the anomalous voltage detector includes a comparator which switches an output level thereof when the charging voltage of the charger is a predetermined value or more.
In such a configuration, a reference voltage of the comparator is set so as to become slightly higher than the charging voltage of the charger employed during normal operating conditions. If the charging voltage of the charger has increased as a result of flow of the anomalous current into the piezoelectric elements, the comparator outputs a signal while being reversed from a high level to a low level or vice versa.
According to the invention, there is also provided a liquid jetting apparatus, comprising:
a liquid jetting head, provided with nozzle orifices;
piezoelectric elements, associated with the nozzle orifices to eject liquid droplets therefrom;
a head drive circuit, which selectively drives the piezoelectric elements;
a charger, which applies a bias voltage to ground-side electrodes of the piezoelectric elements; and
an anomalous voltage detector, which outputs a detection signal when a charging voltage of the charger is a predetermined value or more.
According to the invention, there is also provided a method of driving a liquid jetting head provided with piezoelectric elements which are selectively driven to eject liquid droplets, the method comprising steps of:
applying a bias voltage to ground-side electrodes of the piezoelectric elements by a charger;
detecting whether a charging voltage of the charger is predetermined value or more; and
outputting an anomalous signal when it is detected that the charging voltage is predetermined value or more.
Preferably, the anomalous signal is outputted through a switching element which changes a conduction state thereof when it is detected that the charging voltage is predetermined value or more.
Alternatively, it is preferable that the anomalous signal is outputted through a comparator which switches an output level thereof when it is detected that the charging voltage is predetermined value or more.